Despite the development of science and technology, further efforts are still required in the wireless communication technologies relating to millimeter wave (mmWave). In general, the first challenge is that the wave energy may be significantly attenuated during transmission of the mmWave. The attenuation is closely related to the high frequency band at which a mmWave communication system operates and a rather large bandwidth required for communication in the mmWave communication system. More specifically, compared with the third generation (3G) or the fourth generation (4G) communication system commonly used nowadays, the mmWave communication system adopts a relatively higher frequency band for communication. It is known that an intensity of an electromagnetic wave energy received by a receiver is negatively proportional to a square of a signal transmission distance and is positively proportional to a wavelength of an electromagnetic signal. Therefore, the degree to which the signal energy of the mmWave communication system attenuates is significantly increased because of the high frequency signal with a shorter wavelength adopted in the mmWave communication system. In addition, the use of the high frequency signal also results in a drastic decrease in antenna aperture, and may also result in a decrease in the signal energy for signal transmission in the mmWave communication system. Therefore, to ensure the communication quality, a transceiver in the mmWave communication system normally requires a multi-antenna beamforming technology to reduce signal energy attenuation and thus facilitate the performance of signal transmission and reception.
Generally speaking, the multi-antenna beamforming technology includes arranging an antenna array including a plurality of antennas in a base station/user apparatus and controlling the antennas so that the base station/user apparatus may generate a directional beam. The beamforming technology achieved with the antenna array is crucial to the performance of the mmWave communication system. It is common to adopt a Butler Matrix to control beamformed signals of an antenna array. However, the Butler Matrix is only able to control the directionality of beams in a two-dimensional space, such as controlling a horizontal direction of the beamformed signals. However, a Butler Matrix only capable of controlling the horizontal direction is insufficient for a case where a transmitting end has a difference in height, for example.